Chronic exposure to air pollution can cause harm to cognitive performance, a new study reveals.
Researchers believe that the negative impact increases with age, and affects men with less education the worst.
Over four years, the maths and verbal skills of some 20,000 people in China were monitored by the US-Chinese study.
Researchers believe the results have global relevance, with more than 80 percent of the world's urban population breathing unsafe levels of air pollution.
The study was based on measurements of sulphur dioxide, nitrogen dioxide and particulates smaller than 10 micrometres in diameter where participants lived. It is not clear how much each of these three pollutants is to blame.
Carbon monoxide, ozone and larger particulates were not included in the study.
Described as an invisible killer, air pollution causes an estimated 7 million premature deaths a year worldwide, according to the World Health Organization.
"We provide evidence that the effect of air pollution on verbal tests becomes more pronounced as people age, especially for men and the less educated," the study published in the Proceedings of the National Academy of Sciences reported.
Pollution also increased the risk of degenerative diseases such as Alzheimer's and other forms of dementia, the study suggested.
Exposure to high levels of polluted air "can cause everyone to reduce their level of education by one year ... which is huge," one of the co-authors, Xi Chen of the Yale School of Public Health, told the Guardian.
In this study, researchers tested people of both sexes aged 10 and above between 2010 and 2014, with 24 standardised maths questions and 34 word-recognition questions.
Previous studies found air pollution had a negative impact on students' cognitive abilities.
"This latest study matches with previous research," said Derrick Ho of Hong Kong Polytechnic, who has worked on the health effects of extreme weather events like haze.
"What's new in this paper is the focus on the China scenario and the fact that it's a very detailed study compared to many other ones," he said.
"Also the differentiation between gender and age in this detail is new."
Many pollutants are thought to directly affect brain chemistry in a variety of ways - for instance, particulate matter can carry toxins through small passageways and directly enter the brain.
Some pollutants can also have a psychological impact, increasing the risk of depression.
One of the reasons the researchers suggested older men with less education were worst affected by chronic exposure to air pollution was because those subjects often work outdoor manual jobs.
"Our findings about the damaging effect of air pollution on cognition," the study reported, "particularly on the aging brain, imply that the indirect effect on social welfare could be much larger than previously thought."
The study suggested that while the research findings were specific to China, it could shed light on other developing countries with severe air pollution.
The authors point to the 98 percent of cities with more than 100,000 people in low and middle-income countries that fail to meet WHO air quality guidelines.
Air pollution around the world (stats by World Health Organization)
- About 7 million people die every year from exposure to polluted air
- Air pollution caused 4.2 million deaths globally in 2016
- Nintey-one percent of the world's population lives in places where air quality exceeds WHO guideline limits
- Fourteen Indian cities are among the world's 20 most polluted, with the northern city of Kanpur topping the list
- Nine out of 10 people in the world breathe polluted air